Method of producing homogeneous rods of semiconductor material

ABSTRACT

A PERPENDICULAR TUBE OF SEMICONDUCTOR MATERIAL IS FILLED WITH CRYSTALLINE SEMICONDUCTOR WASTE AND/OR DOPANT. A MOLTEN ZONE IS GUIDED THROUGH THE TUBE AND THE FILLING, WHEREBY THE FILLING FUSES WITH THE TUBE.

Dec. 25, 1973 REUSCHEL ETAL METHOD OF PRODUCING HOMOGENEOUS RODS OFSEMICONDUCTOR MATERIAL Filed Dec. 2, 1971 Fig.1 16 4 I q i g x 5 Fig.2

g 2g "20 11 Z1 2 1 flu United States Patent US. Cl. 25262.3 R 8 ClaimsABSTRACT OF THE DISCLOSURE A perpendicular tube of semiconductormaterial is filled with crystalline semiconductor waste and/or dopant. Amolten zone is guided through the tube and the filling, whereby thefilling fuses with the tube.

The present invention relates to a method for producing, by melting,homogeneous rods of semiconductor material.

Such methods are widely known. In one of these methods, thesemiconductor material is first pyrolytically precipitated from agaseous compound, upon heated rod shaped carrier bodies. The carrierbodies are usually of the same semiconductor material and may be more orless heavily doped. Following precipitation of semiconductor material, arod is obtained whose interior is doped, but whose outer layers are notdoped. The dopant is then uniformly distributed by means of the known,cruciblefree zone melting process so that a low doping is now uniformlyestablished over the entire cross section. A plurality of rod shapedcarrier bodies can be produced from this rod, e.g. by pulling thin.Semiconductors can be precipitated from the gaseous phase, upon saidcarrier bodies. This process, which comprises the alternate steps ofprecipitation and zone melting, may be continued until the desiredhomogeneous doping is obtained. However, the method is relativelycumbersome, and has the additional disadvantage that cutoff parts ofsemiconductor material cannot be used.

The cutoff or waste pieces are, therefore, molten in a crucible and arod is pulled therefrom, by the Czochralski method. The dopant necessaryfor the basic doping can also he placed into the crucible and becomesinstalled into the rod, during the pulling from the melt. The cruciblemethod has the disadvantage, however, that the impurities which arepresent in the crucible will also become installed in the rod. Thus, forexample, oxygen will penetrate to a considerable degree, into thesemiconductor material when the crucible is made of quartz.

It is an object of the present invention to provide a method of theaforedescribed type, which makes it possible to utilize the Wastematerial and/or to obtain the desired doping in a simple manner, withouthaving to accept the above-indicated shortcomings.

The invention is characterized by the fact that a substantiallyperpendicularly positioned tube of semiconductor material, which is heldat both ends and has a sealed bottom, is provided with a filling ofcrystalline semiconductor pieces and optionally with doping material.The term perpendicularly used herein and in the claims also includesubstantially perpendicular. A molten zone is introduced through saidtube into the filling and the parts lying on both sides of the moltenzone are rotated, relative to one another, during the melting. Thefilling may consist of doped or undoped pieces of semiconductormaterial. If a filling, consisting of undoped pieces of semiconductormaterial is used, it is recom- 3,781,209 Patented Dec. 25, 1973 mendedto place a rod which contains dopant, into the tube. This rod is atleast almost as long as the tube. A rod containing only dopant and beingat least almost of the same length as the tube, may also be insertedinto said tube. The rod is, preferably, conical. The melting zone can bealso moved from the melting point of a monocrystal, through the tube andthe filling. Thus, the semiconductor rod may be transformed into amonocrystal, while simultaneously obtaining homogeneous quali ties.

The invention will be disclosed in greater detail, with reference to thedrawing, wherein:

FIG. 1, schematically, illustrates partly in section one embodiment ofthe invention; and

FIG. 2, schematically, illustrates partly in section a secondembodiment.

The device of FIG. 1 has a vacuum tight housing 1. Two holders 2 and 3,between which a tube 8 of semiconductor material is clamped, areprovided in the housing 1. The holder 2 has screws 4, which hold thetube 8 in its position. Holder 2 is connected with a shaft which leadsthrough a packing seal 5 to a drive unit 16. The shaft 19 may be rotatedby the drive unit as well as be shifted along its axis. This isindicated by the arrows. In a similar manner, holder 3 is connected to adrive unit 17, via a shaft 18, that is led through packing seal 6. Thedrive unit 17 rotates the shaft 18 into rotation and shifts it along itsaxis. The device also contains a high frequency coil 12, which isconnected via a lead 14 through packing 15, with a high frequencygenerator (not shown), positioned outside the housing 1. The coil 12produces, with the aid of HF energy, a melting zone 10'. A part 11 ofthe rod to be produced is seen above the molten zone 10. Below themolten zone 10 is a portion 13 of the original tube. This portion 13 isfilled with pieces 9 of semiconductor material, such as waste, forexample.

The method is performed by first placing the high frequency coil :12near the upper end of the tube 8. A part of the tube and of the fillingis then molten, at the same time, parts 11 and 13 are turned,oppositely, relative to each other, at a r.p.m. of 5 to 100, preferablyabout 40 r.p.m. This provides a homogeneous distribution of the materialin the rod being produced. If, for example, the tube 8 consists ofundoped material and if pieces of doped semiconductor material arelocated among the filling 9, the rod will obtain a homogeneousdistribution and an appropriate quantity of doping. According to thespace filling degree of the filling 9 which is smaller than 1, theholders 2 and .3 must be moved toward each other from the top downwardas the molten zone advances so that the diameter of the rod remainsequal over the entire length. To this end, either the holders 2, 3 maybe moved independently or both holders may be moved at the same time.This is effected by drives 16 or 17 or 16 and 17. The distance throughwhich the holders must be moved, can be determined by a regulator, notshown in the drawing, which regulates the melting zone 10 to a constantvolume. This type of regulating method is described, for example, inGerman Pat. No. 1,153,908. Here, the high frequency coil is a componentof an oscillatory or resonant circuit, whose inductivity isfundamentally determined by the volume of the melting zone and by thecoil, itself. A frequency impressed upon the resonant circuit, lines inthe slope of the resonance frequency of the resonant circuit. If theinductivity of the resonance circuit is altered through an increase or adecrease in the volume of the melting zone, the resulting voltage changeis used as a controlled quantity for shifting the holders 2, 3, via thedrive units 16 and 17, respectively. This type of control ensures thatthe volume of the melting zone remains constant and that the rod to beproduced, has the same cross section, at each location.

If the tube and the filling are made of silicon, a melting temperatureof about 1420 C. is established. The tube may be 20 to 80 mm. thick andhave a wall thickness of 1 to 5 mm. The speed of the melting zone ispreferably 0.5 to 5 mm. per minute.

The method was described with reference to an embodiment which shows themelt being moved from the top down. It is also possible to move themelt, from below upward, through the tube and the filling. This createsa rod which issues from below, while the tube, respectively, the remnantof the tube is situated above the melt. The tube can be efficiently heldby a holding device so that the filling 9 does not sink into the melt;as it must be firmly pounded or pressed into the tube.

FIG. 2 shows another embodiment of the invention. Only the essentialparts are illustrated, while the drive members, the housing, etc. wereomitted for the sake of simplicity. The device has a tube 20, which isheld in holder 26 by two screws 27. A core 21 which contains dopingmaterial is situated in the tube 20. The core 21 is positioned in theholder 20 by screws 28. The tube may also contain a filling 29 of dopedand undoped pieces of semiconductor material. The melting coil of thisexample was given numeral 22; the molten zone numeral 23 and the rodshaped part numeral 24. A crystal 25 with a small diameter which isfused to the lower end of the rod shaped part 24 may, for example, be amonocrystal.

In this embodiment, the melting zone is led through the tube, from belowupward. Since the distribution coefiicient of most dopants, such as, forexample, gallium, arsenic, antimony, is less than 1, meaning that lessdopant is dissolved in solid semiconductor material than in liquidsemiconductor material, this would cause the doping of the rod toincrease constantly from below, upward. The dopant core 21 is,therefore, constructed in the shape of a cone, with a diameter ratiofrom below to the top of 3:1 to 2:1. The dopant core consists,preferably, of alloys of semiconductor material with the dopant, such asof Si/P, Si/As, Si/Sb, Si/Ga, etc. The allowing ratio may be variedwithin wide scopes, according to the level of the basic doping that isalready present in the semiconductor rod and according to the weightratio, between core and rod.

If boron is used for doping, a core comprising an Si/B alloy will beused and will not be conically shaped, since the distributioncoefiicient of boron in silicon, is approximately equal to 1.

In FIG. 2, the core is shown positioned concentrically to the tube. Thisis not necessary, however, since zone melting produces a mixing effect,which results in a uniform distribution of the material in the rod.

What is claimed is:

1. A method of producing rods of semiconductor material by zone melting,which comprises holding, perpen dicularly to the surface of the earth atboth ends, a tube of semiconductor material, having a sealed bottom,said tube being filled with a filling of crystalline semiconductormaterial pieces and dopant, passing a melting zone comprisingsemiconductor material through said tube and filling while rotating thepart of the tube above the melting zone relative to the part of the tubebelow the melting zone during the melting process, the tube and thefilling being of the same material and being part of the melting zone,and the melting zone being moved over the entire length of the tube.

2. The method of claim 1, wherein a filling of doped semiconductorpieces is within said tube.

3. The method of claim 1, wherein a filling of nondoped semiconductorpieces is within said tube.

4. The method of claim 3, wherein a core is inserted into the filling,said core containing dopant and being of almost as long as said tube.

5. The method of claim 4, wherein the core is conical and has a diameterratio from bottom to top of 3 :1 to 2:1.

6. The method of claim 1, wherein the melting zone is moved through thetube and the filling from the fusion point of a monocrystal locatedbelow the tube.

7. The method of claim 1, wherein the molten zone is moved from the topdown through the filling and the tube.

8. The method of claim 1, wherein the molten zone is moved from thebottom up through the tube and the filling.

References Cited UNITED STATES PATENTS 3,141,848 7/1964 Emk et al25262.3 E

FOREIGN PATENTS 1,113,682 9/1961 Germany 23301 S P OSCAR R. VERTIZ,Primary Examiner J. COOPER, Assistant Examiner US. Cl. X.R.

252-62.3 E; 2330l SP; 1481.6

